Nigeria's presidential election, postponed until 28 March, promises to be a closely fought rematch between incumbent Goodluck Jonathan and former military ruler Muhammadu Buhari.

The rise of Islamist group Boko Haram in the north-east has put security at the centre of their election campaigns, but the candidates are at odds over how to handle the insurgency in Africa's biggest oil producer. Boko Haram has recently pledged allegiance to Islamic State.

The elections were postponed for six weeks, just a week before they were originally due to be held in mid-February.

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Violence erupted in the northern state of Gombe ahead of an election rally by President Jonathan

The Independent National Electoral Commission (Inec) said this was because the military had advised it would be unable to provide security as its soldiers were committed to the fight against Boko Haram.

Past elections have been marred by violence and allegations of vote-rigging. Since campaigning began in mid-November, both the ruling and opposition camps have reported violent attacks which have killed a number of their supporters.

Who are the main candidates?

Fourteen candidates are contesting the election but only Mr Jonathan and Gen Buhari have a realistic chance of winning.

President Jonathan is seeking a second four-year term. His People's Democratic Party (PDP) has dominated Nigerian politics since civilian rule was restored in 1999 but now faces its toughest election challenge, from the All Progressives Congress (APC).

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Supporters of Goodluck Jonathan turned out in large numbers for a rally in Port Harcourt, south Nigeria

Mr Jonathan is expected to do well on his home ground in the mainly Christian south. But his government has been fiercely criticized for its failure to combat Boko Haram in the north-east.

President Jonathan told the BBC on 20 March that Boko Haram was "getting weaker and weaker every day" and that their territory could be recaptured within a month.

The government claims that major gains have been made with the help of Cameroon, Chad and Nigeria.

Gen Buhari has lost the last three elections but some sections of the Nigerian media are predicting a win this time. He has described the PDP's 16-year rule as "a disaster for the country and its citizens".

He is said to be extremely popular in the mainly Muslim north and has in the past supported the implementation of Islamic law there.

Gen Buhari has made security a priority during his presidential election campaign. He has promised to crush the Islamist insurgency within months if elected.

He has publicly denounced Boko Haram repeatedly, branding them "bigots masquerading as Muslims." Last July, he survived an attack on his convoy allegedly carried out by the group.

Fears are rife that Boko Haram's insurgency may disenfranchise voters in the north, seen as a Buhari stronghold.

Gen Buhari is also expected to do well in the south-west around the commercial capital Lagos.

But former militants in the oil-rich southern Niger Delta have endorsed Mr Jonathan's candidacy and warned of violence if Gen Buhari wins. The APC party has condemned their threats as "barbaric".

Where they stand on key election issues

While the personalities of the two main candidates have been at the forefront of this election campaign, certain key issues - namely insecurity, elite corruption among high-profile politicians and business leaders and the state of the economy - have become increasingly important to voters.

Goodluck Jonathan

Muhammadu Buhari

Security

Says Boko Haram can be defeated in April. Seeks greater regional and international cooperation to tackle the insurgency, terrorism, piracy and organised crime.

Says the government has been ineffective and lacks the willpower to fight Boko Haram. Pledges to end the insurgency within months if elected.

Economy

Says he will continue with his economic blueprint known as the "2011-2015 Transformation agenda". Views economic diversification as a key step towards addressing the fall in global oil prices.

Says government's economic policies have worsened the lives of Nigerians. Promises to pick "competent hands" to run the economy. Pledges to tackle poverty by closing the wealth gap through shared economic growth.

Corruption

Says "we are fighting corruption. It is not by publicly jailing people. Yes, we believe in suppressing corruption, but our emphasis is in prevention."

Says one of his key priorities is to wipe out corruption. "If Nigeria doesn't kill corruption, corruption will kill Nigeria".

Employment

Promises to create 2 million jobs each year. Launched YouWIN scheme for young entrepreneurs and Sure-P initiative aimed at helping graduates find jobs.

Promises to create 20,000 jobs per state, totalling 720,000. Pledges support for the agricultural sector and soft loans for small manufacturers to boost job creation.

Infrastructure

Credits his administration with reviving the railway system and improving road infrastructure.

Pledges to complete stalled road projects and improve infrastructural development nationwide, especially in the north-east.

Energy

Says government's privatization of the power generation and distribution companies will ensure regular power supplies in the future.

Says he will tackle a sector "riddled with corruption and mismanagement" and adopt a market-based approach. Favours exploration of non-oil sector.

How does the electoral system work?

The Independent National Electoral Commission (Inec) has promised a clean ballot. All 14 candidates have signed an agreement binding them to credible and non-violent elections. Official campaigning is due to end on 27 March - 24 hours before polling day.

The candidate with the most votes is declared the winner in the first round, as long as he gains at least 25% of the votes in two-thirds of Nigeria's 36 states.

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Permanent Voters' Cards are being used for the first time in Nigeria

Biometric cards will be used for the first time. Inec says more than 80% of the nearly 70 million eligible voters have obtained their identity cards. The minimum voting age is 18.

An extra 30,027 polling stations have been set up, bringing the total to 150,000 nationwide.

Polls will open at 08:00 local time (07:00 GMT). All voters must be present at their designated polling station by 13:00 local time to be allowed to cast their ballot. Polls will close when the last person in the queue has voted.

The authorities say 360,000 police officers will be deployed at strategic areas, along with sniffers dogs.

Inec has approved the presence of international and local observers to monitor the elections, although the European Union says its observers will not deploy in the north-east due to security concerns.

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Women fleeing Boko Haram violence queue at a camp for displaced people in Borno State

Parliament amended the electoral law on 15 January, allowing an estimated one million people displaced by the insurgency to cast their votes. They can vote at specially-provided facilities near or within camps in their states of origin.

When will we get the results?

In Nigeria, as in many parts of Africa, exit polls are rarely used to call elections as results are so often disputed.

Once votes have been counted, the results from each polling unit will be uploaded to the electoral commission's website. Inec says it expects to announce the final results within 48 hours.

In reality though, the final outcome may not be known for some time.

And judging by previous elections, it is likely that the results will be challenged.

What happens if there is a run-off?

If there is no outright winner in the first round, the law states a run-off election must be held within seven days. But Inec has said it is doubtful whether a run-off vote could be organized in a week. Victory in a run-off election is by simple majority.

What about the parliamentary and gubernatorial elections?

Parliamentary elections have also been put back to 28 March, with 739 candidates vying for a place in the 109-seat Senate and 1,780 seeking election to the 360-seat National Assembly.

Nigerians will vote again on 11 April to choose new governors and state assemblies for 29 of the 36 states.

Like the president, governors are limited to two four-year terms, so this election will see new occupants in many states.

Governors hold huge sway because they allocate federally disbursed revenue and shape policy on development and security in their states.

Among the key states to watch are Lagos, Kano and Rivers - currently in APC hands - because of their large populations and economic power.

Some of these states have budgets larger than those of neighbouring countries, meaning there is fierce competition to run them.